Yes, the date and the time. I'm not sure how quickly the camera clock drifts - I bet it doesn't keep time that well

Did a quick check. I got my D800e about a month and 3 weeks ago and immediately set the date and clock as everybody does. It was set to match the computer clock within the minute (the computer itself uses a time server to keep its clock precise).

The camera is still within the minute of the computer time. So at most the drift is less than a minute after a month and 3 weeks (unfortunately I don't have a more accurate assessment). This translates in less than a 8 minute drift a year. That's more than enough accuracy for what I need for this purpose.
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Thierry